Conventionally, it is known that films and various by-products made of polyethylene naphthalate, which is formed by reacting 2,6-naphthalenedicarboxylic acid and ethylene glycol, have improved mechanical strength, heat-resistance, size stability etc. relative to those produced from polyethylene terephthalate which is formed from terephthalic acid.
As for the production of 2,6-naphthalenedicarboxylic acid (hereinafter referred to as 2,6-NDA), the following methods are known. (A) Methods of producing 2,6-NDA by the oxidation of dialkyl-substituted naphthalene with molecular oxygen in an acetic acid solvent in the presence of a catalyst comprising cobalt, manganese and bromine (Japanese Publication for Examined Patent applications No. 48-27318/1973, No. 56-3337/1981, Japanese Publication for Unexamined Patent applications No. 61-140540/1986, No. 62-212345/1987, No. 64-3148/1989, No. 1-160943/1989 and No. 1-287055/1989).
Meanwhile, for the production of aromatic carboxylic acids, (B) a method disclosed in Japanese Publication for Unexamined Patent Application No. 52-77022/1977 and (C) a method disclosed in Japanese Publication for Examined Patent Application No. 60-56694/1986 are known.
In methods (A), when the reaction is started, a large amount of catalyst in proportion to the starting material is required in order to repress the formation of undesirable by-products, including tar-like substances and naphthalene ring-scissioned by-products such as trimellitic acid, and additionally to improve the yield of 2,6-NDA. Consequently, complicated industrial processes are required in these methods in order to separate and recover the catalyst after the reaction. Moreover, in order to obtain 2,6-NDA of a high purity, a number of refining operations are necessary.
In method (B), it is disclosed that in producing terephthalic acid with a catalyst comprising cobalt, manganese and bromine in an acetic acid solvent, if a small amount (ppm) of copper is added to the acetic acid solvent, the oxidative decomposition of the acetic acid solvent is repressed. In this method, however, adding copper does not stimulate the catalytic reaction, and therefore it is hard to believe that copper contributes to the catalytic reaction. In method (C) for producing terephthalic acid with a catalyst comprising copper and bromine in a water solvent, the highest yield of terephthalic acid (molar yield) is around 70 percent, and again no statement on 2,6-NDA is disclosed in this application.
As is clear from the prior art, a method of producing 2,6-NDA from dialkyl-substituted naphthalene at a high yield has not yet been fully established.
Meanwhile, diaryldicarboxylic acids are important compounds as copolymer components for the manufacture of fibers, films, plasticizers, synthetic resins etc.
Conventionally, the production of the diaryldicarboxylic acids by the oxidation of starting material with molecular oxygen in an acetic acid solvent in the presence of a catalyst comprising cobalt, manganese and bromine is known. For this kind of method, for instance, the following four methods are known: (1) A method of producing diaryldicarboxylic acids by the oxidation of 4,4'-dimethylbiphenyl (see Zh. Prikl. Khim.40 (4), 935-6 (1967)); (2) A method of producing diaryldicarboxylic acids by the oxidation of 4,4'-dimethylbiphenyl (see Japanese Publication for Unexamined Patent Applications No. 2-32041/1990 and No. 63-63638/1988); (3) A method of producing diaryldicarboxylic acids by the oxidation of 4,4'-diisopropylbiphenyl (see Japanese Publication for Unexamined Patent Application No. 63-122645/1988); and (4) A method of producing diaryldicarboxylic acids by the oxidation of 4,4'-dicyclohexylbiphenyl (see Japanese Publication for Unexamined Patent Application No. 57-16831/1982).
Also, (5) Japanese Publication for Unexamined Patent Application No. 63-310846/1988 discloses a method of producing various kinds of diaryldicarboxylic acids in the presence of the above-mentioned catalyst.
However, these methods present the following drawbacks. In method (1), 4,4'-biphenyldicarboxylic acids can be obtained at 79 mole percent yield by oxidizing 4,4'-dimethylbiphenyl with a catalyst comprising cobalt, manganese and bromine in an acetic acid solvent. However, the amount of the high-cost cobalt catalyst required is equivalent to 20 weight percent of the amount of the starting material, thereby resulting in a high production cost. Besides, 79 mole percent yield is not high enough.
In method (2), the reaction is carried out in an acetic acid solvent in the presence of a catalyst comprising cobalt, manganese and bromine, in order to obtain 4,4'-biphenyldicarboxylic acid at a high yield of at least 80 mole percent. However, since the amount of the high-cost cobalt catalyst required is equivalent to 15 weight percent of the amount of a substrate, this method also results in a high production cost.
In method (3), the reaction is carried out in an acetic acid solvent in the presence of a catalyst comprising cobalt, manganese and bromine equivalent to at least 15 weight percent of the amount of the starting material. This method achieves only a low yield of 35.8 mole percent.
In method (4), the reaction is carried out in the presence of a catalyst comprising similar catalyst components to the catalysts in methods (1), (2), (3), whose weight ratio to the starting material is at least 30 percent. However, this method also results in a low yield of 40 mole percent.
In method (5), diaryldicarboxylic acids are produced at a high yield of at least 90 mole percent by oxidizing various kinds of dialkyl-substituted diaryl compounds in the presence of a catalyst whose essential components are cobalt and bromine. However, the present inventors examined this method and found that the products had dark color. The following two reasons are listed for this cause: firstly, due to the cobalt catalyst; and secondly, the formation of large amounts of by-products which easily to color and of tar-like substances. Moreover, since the high-cost cobalt catalyst is essential in this method, industrially its production cost is not sufficiently low.
As aforesaid, in the above conventional methods, as large amounts of catalyst comprising high-cost cobalt catalyst are used, industrially sufficiently low production costs can not be achieved. Also, a method of producing diaryldicarboxylic acids having light color from the dialkyl-substituted diaryl compounds in the presence of the above-mentioned conventional catalysts with high yields of diaryldicarboxylic acids has not yet been established.